Fig. 4
Mechanism of upconverted emission in the two-particle picture. The solid lines represent the excitonic ground-state dispersion n1 of the IX (gray), MoS2 (blue) and WSe2 (red). The circles stand for excitons. Q′ is the momentum mismatch as defined in Fig. 1a. The dashed lines indicate excited states n and the shaded area marks the excitonic continuum n∞. The gray-scale shading of circles schematically pictures the momentum distribution of excitons. A bright shading indicates less excitons for a given momentum than a dark shading. The photon dispersion is overlaid (gray lines) to mark the region of effective radiative recombination (orange circles). For the situation depicted, the bias voltage is below the threshold for direct intralayer charge injection. Mechanism (I) illustrates radiative IX emission facilitated by an increasing number of IX with large momenta. Mechanism (II) depicts excitonic Auger processes. The gray ellipse schematically highlights the interaction between two exemplary excitons. As a result of the interaction, one exciton recombines non-radiatively and transfers the energy to the other exciton (arrows with dotted lines). Relaxation: (i) describes relaxation back to the IX ground state (exciton–exciton annihilation), (ii) and (iii) relaxation to MoS2 and WSe2, respectively, which leads to upconverted intralayer emission
